Barrier film for food packaging

ABSTRACT

Barrier films are prepared from a blend of two high density polyethylene blend components and a high performance organic nucleating agent. The two high density polyethylene blend components have substantially different melt indices. Large reductions in the moisture vapor transmission rate of the film are observed in the presence of the nucleating agent when the melt indices of the two blend components have a ratio of greater than 10/1. The resulting barrier films are suitable for the preparation of packaging for dry foods such as crackers and breakfast cereals.

REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/856,627 filed on Apr. 4, 2013 and is a continuation of U.S. patentapplication Ser. No. 11/983,284 filed on Nov. 8, 2007.

FIELD OF THE INVENTION

This invention relates to barrier films which are prepared from a blendof at least two high density polyethylene (hdpe) resins and a nucleatingagent. The films are used to prepare packaging for dry foods such ascrackers and breakfast cereals.

BACKGROUND OF THE INVENTION

Polyethylene may be classified into two broad families, namely “random”(which is commercially prepared by initiation with free radicals underpolymerization conditions that are characterized by the use of very highethylene pressures) and “linear” (which is commercially prepared with atransition metal catalyst, such as a “Ziegler Natta” catalyst, or a“chromium” catalyst, or a single site catalyst or a “metallocenecatalyst”).

Most “random” polyethylene which is commercially sold is a homopolymerpolyethylene. This type of polyethylene is also known as “high pressurelow density polyethylene” because the random polymer structure producesa lower polymer density. In contrast, most “linear” polyethylene whichis commercially sold is copolymer of ethylene with at least one alphaolefin (especially butene, hexene or octene). The incorporation of acomonomer into linear polyethylene reduces the density of the resultingcopolymer. For example, a linear ethylene homopolymer generally has avery high density (typically greater than 0.955 grams per cubiccentimeter (g/cc))—but the incorporation of small amounts of comonomerresults in the production of so-called “high density polyethylene” (or“hdpe”—typically, having densities greater than 0.935 g/cc) and theincorporation of further comonomer produces so-called “linear lowdensity polyethylene” (or “lldpe”—typically having a density of fromabout 0.905 g/cc to 0.935 g/cc).

Some plastic film is made from hdpe. One particular type of hdpe film isused to prepare food packaging with “barrier properties”—i.e. the filmacts as a “barrier” to water vapor transmission. This so-called “barrierfilm” is used to prepare packages (or liners for cardboard packages) forbreakfast cereals, crackers and other dr foodstuffs.

It has recently been discovered that the barrier properties of hdpe filmmay be improved by the addition of a nucleating agent.

We have now discovered that further improvements in barrier propertiesmay be achieved by the use of a blend of two hdpe resins which havesubstantially a different melt index from each other.

SUMMARY OF THE INVENTION

The present invention provides:

I) an organic barrier nucleating agent; andII) a high density polyethylene blend composition comprising:

-   -   II-i) from 5 to 60 weight % of at least one high density        polyethylene blend component a) having a high melt index, I₂;        and    -   II-ii) from 95 to 40 weight % of at least one high density        polyethylene blend component b) having a low melt index, I₂′,        wherein:        -   w) said organic barrier nucleating agent is added in an            amount of from 100 to 3000 parts per million based on the            weight of said high density blend composition;        -   x) each of said blend component a) and blend component b)            has a density of from 0.950 to 0.975 g/cc;        -   y) the melt index, I₂, of said blend composition is from 0.5            to 10 grams/10 minutes; and        -   z) the I₂ ratio, obtained by dividing the I₂ value of said            blend component a) by the I value of said blend component b)            is greater than 10/1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Barrier Film and FoodPackaging

Plastic films are widely used as packaging materials for foods. Flexiblefilms, including multilayer films, are used to prepare bags, wrappers,pouches and other thermoformed materials.

The permeability of these plastic films to gases (especially oxygen) andmoisture is an important consideration during the design of a suitablefood package.

Films prepared from thermoplastic ethylene-vinyl alcohol (“EVOH”)copolymers are commonly employed as an oxygen barrier and/or forresistance to oils. However, EVOH films are quite permeable to moisture.

Conversely, polyolefins, especially high density polyethylene, areresistant to moisture transmission but comparatively permeable tooxygen.

The permeability of linear polyethylene film to moisture is typicallydescribed by a “water vapor transmission rate” (or “WVTR”). In certainapplications some vapor transmission is desirable—for example, to allowmoisture out of a package which contains produce. The use of linear lowdensity polyethylene (lldpe) which may be filled with calcium carbonate(to further increase vapor transmission) is common for this purpose.

Conversely, for packages which contain crispy foods such as breakfastcereals or crackers, it is desirable to limit WVTR to very low levels toprevent the food from going stale. The use of hdpe to prepare “barrierfilm” is common for this purpose. A review of plastic films and WVTRbehavior is provided in U.S. Pat. No. 6,777,520 (McLeod et al.)

This invention relates to “barrier films” prepared from hdpe—i.e. filmswith low MVTR. As will be appreciated from the above description of EVOHfilms, it is also known to prepare multilayer barrier films to produce astructure which is resistant to moisture and oxygen. Multilayerstructures may also contain additional layers to enhance packagingquality—for example, additional layers may be included to provide impactresistance or sealability. It will also be appreciated by those skilledin the art that “tie layers” may be used to improve the adhesion between“structural” layers. In such multilayer structures, the hdpe barrierlayer may either be used as an internal (“core”) layer or external(“skin”) layer.

The manufacture of “barrier” food packaging from plastic resins involvestwo basic operations.

The first operation involves the manufacture of plastic film from theplastic resin. Most “barrier films” are prepared by “blown film”extrusion, in which the plastic is melted in an extruder, then forcedthrough an annular die. The extrudate from the annular die is subjectedto blown air, thus forming a plastic bubble. The use of multipleextruders and concentric dies permits multilayer structures to beco-extruded by the blown film process. The “product” from this operationis “barrier film” which is collected on rolls and shipped to themanufacturers of food packaging.

The manufacturer of the food packaging generally converts the rolls ofblown film into packaged foods. This typically involves three basicsteps:

1) forming the package;

2) filling the package;

3) sealing the food in the finished package.

Although the specific details will vary from manufacturer tomanufacturer, it will be readily appreciated that the film needs to havea balance of physical properties in order to be suitable for foodpackaging. In addition to low MVTR, it is desirable for the film to“seal” well and to have sufficient impact strength and stiffness (orfilm “modulus”) to allow easy handling of the package. Multilayercoextrusions are often used to achieve this balance of properties, with3 and 5 layer coextrusions being well known. Sealant layers may beprepared with ethylene—vinyl acetate (EVA) ionomers (such as those soldunder the trademark SURLYN™ by E.I. DuPont), very low densitypolyethylene (polyethylene copolymers having a density of less than0.910 grams per cubic centimeter) and blends with small amounts ofpolybutene. It is known to use sealant compositions in both “skin”layers of a coextrusion or in only one of the skin layers.

HDPE Blend Components and Overall Composition

The plastic used in the barrier film of this invention is high densitypolyethylene (hdpe). Specifically, the hdpe must have a density of atleast 0.950 grams per cubic centimeter (“g/cc”) as determined by ASTM D1505. Preferred hdpe has a density of greater than 0.955 g/cc and themost preferred hdpe is a homopolymer of ethylene.

Blend Components

Blend Component a)

Blend component a) of the polyethylene composition used in thisinvention comprises an hdpe with a comparatively high melt index. Asused herein, the term “melt index” is meant to refer to the valueobtained by ASTM D 1238 (when conducted at 190° C., using a 2.16 kgweight). This term is also referenced to herein as “I₂” (expressed ingrams of polyethylene which flow during the 10 minute testing period, or“gram/10 minutes”). As will be recognized by those skilled in the art,melt index, I₂, is in general inversely proportional to molecularweight. Thus, blend component a) of this invention has a comparativelyhigh melt index (or, alternatively stated, a comparatively low molecularweight) in comparison to blend component b).

The absolute value of I₂ for blend component a) is preferably greaterthan 5 grams/10 minutes. However, the “relative value” of I₂ for blendcomponent a) is critical—it must be at least 10 times higher than the I₂value for blend component b) [which I₂ value for blend component b) isreferred to herein as I₂]. Thus, for the purpose of illustration: if theI₂′ value of blend component b) is 1 gram/10 minutes, then the I₂ valueof blend component a) must be at least 10 grams/10 minutes.

Blend component a) is further characterized by:

i) density—it must have a density of from 0.950 to 0.975 g/cc; and

ii) weight % of the overall polyethylene composition—it must be presentin an amount of from 5 to 60 weight % of the total hdpe composition(with blend component b) forming the balance of the total polyethylene)with amounts of from 10 to 40 weight %, especially from 20 to 40 weight%, being preferred. It is permissible to use more than one high densitypolyethylene to form blend component a).

The molecular weight distribution [which is determined by dividing theweight average molecular weight (Mw) by number average molecular weight(Mn) where Mw and Mn are determined by gel permeation chromatography,according to ASTM D 6474-99] of component a) is preferably from 2 to 20,especially from 2 to 4. While not wishing to be bound by theory, it isbelieved that a low Mw/Mn value (from 2 to 4) for component a) mayimprove the nucleation rate and overall barrier performance of blownfilms prepared according to the process of this invention.

Blend Component b)

Blend component b) is also a high density polyethylene which has adensity of from 0.950 to 0.970 g/cc (preferably from 0.955 to 0.965g/cc).

The melt index of blend component b) is also determined by ASTM D 1238at 190° C. using a 2.16 kg load. The melt index value for blendcomponent b) (referred to herein as I₂′) is lower than that of blendcomponent a), indicating that blend component b) has a comparativelyhigher molecular weight. The absolute value of I₂′ is preferably from0.1 to 2 grams/10 minutes.

The molecular weight distribution (Mw/Mn) of component b) is notcritical to the success of this invention, though a Mw/Mn of from 2 to 4is preferred for component b).

As noted above, the ratio of the melt index of component b) divided bythe melt index of component a) must be greater than 10/1.

Blend component b) may also contain more than one hdpe resin.

Overall HDPE Composition

The overall high density blend composition used in this invention isformed by blending together blend component a) with blend component b).This overall hdpe composition must have a melt index (ASTM D 1238,measured at 190° C. with a 2.16 kg load) of from 0.5 to 10 grams/10minutes (preferably from 0.8 to 8 grams/10 minutes).

The blends may be made by any blending process, such as: 1) physicalblending of particulate resin; 2) co-feed of different hdpe resins to acommon extruder; 3) melt mixing (in any conventional polymer mixingapparatus); 4) solution blending; or, 5) a polymerization process whichemploys 2 or more reactors.

One preferred hdpe blend composition is prepared by melt blending thefollowing two blend components in an extruder:

from 10 to 30 weight % of component a): where component a) is aconventional hdpe resin having a melt index, I₂, of from 15-30 grams/10minutes and a density of from 0.950 to 0.960 g/cc with

from 90 to 70 weight % of component b): where component b) is aconventional hdpe resin having a melt index, I₂, of from 0.8 to 2grams/10 minutes and a density of from 0.955 to 0.965 g/cc.

An example of a commercially available hdpe resin which is suitable forcomponent a) is sold under the trademark SCLAIR™ 79F, which is an hdperesin that is prepared by the homopolymerization of ethylene with aconventional Ziegler Natta catalyst. It has a typical melt index of 18grams/10 minutes and a typical density of 0.963 g/cc and a typicalmolecular weight distribution of about 2.7.

Examples of commercially available hdpe resins which are suitable forblend component b) include (with typical melt index and density valuesshown in brackets):

-   -   SCLAIR™ 19G (melt index=1.2 grams/10 minutes, density=0.962        g/cc);    -   MARFLEX™ 9659 (available from Chevron Phillips, melt index=1        grams/10 minutes, density=0.962 g/cc); and    -   ALATHON™ L 5885 (available from Equistar, melt index=0.9        grams/10 minutes, density=0.958 g/cc).

A highly preferred hdpe blend composition is prepared by a solutionpolymerization process using two reactors that operate under differentpolymerization conditions. This provides a uniform, in situ blend of thehdpe blend components. An example of this process is described inpublished U.S. patent application 20060047078 (Swabey et al.), thedisclosure of which is incorporated herein by reference. The overallhdpe blend composition preferably has a MWD (Mw/Mn) of from 3 to 20.

Nucleating Agents

The term “nucleating agent”, as used herein, is meant to convey itsconventional meaning to those skilled in the art of preparing nucleatedpolyolefin compositions, namely an additive that changes thecrystallization behavior of a polymer as the polymer melt is cooled.

Nucleating agents are widely used to prepare classified polypropyleneand to improve the molding characteristics of polyethylene terphlate(PET).

A review of nucleating agents is provided in U.S. Pat. Nos. 5,981,636;6,466,551 and 6,559,971, the disclosures of which are incorporatedherein by reference.

There are two major families of nucleating agents, namely “inorganic”(e.g. small particulates, especially talc or calcium carbonate) and“organic”.

Examples of conventional organic nucleating agents which arecommercially available and in widespread use as polypropylene additivesare the dibenzylidene sorbital esters (such as the products sold underthe trademark Millad™ 3988 by Milliken Chemical and Irgaclear™ by CibaSpecialty Chemicals). The present invention does not utilize either ofthe above described “inorganic” or conventional organic nucleatingagents because they do not always improve the barrier performance offilms prepared from hdpe resins (as shown in the Examples). Thenucleating agents which are used in the present invention are generallyreferred to as “high performance nucleating agents” in literaturerelating to polypropylene. These nucleating agents are referred toherein as “organic barrier nucleating agents”—which, (as used herein),is meant to describe an organic nucleating agent which improves(reduces) the moisture vapor transmission rate (MVTR) of a film preparedfrom hdpe. This may be readily determined by: 1) preparing an hdpe filmhaving a thickness of 1.5-2 mils in a conventional blown film process(as described in the Examples below) in the absence of a nucleator; 2)preparing a second film of the same thickness (with 1000 parts permillion by weight of the organic nucleator being well dispersed in thehdpe) under the same conditions used to prepare the first film. If theMVTR of the second film is lower than that of the first (preferably, atleast 5-10% lower), then the nucleator is suitable for use in thepresent invention.

High performance, organic nucleating agents which have a very highmelting point have recently been developed. These nucleating agents aresometimes referred to as “insoluble organic” nucleating agents—togenerally indicate that they do not melt disperse in polyethylene duringpolyolefin extrusion operations. In general, these insoluble organicnucleating agents either do not have a true melting point (i.e. theydecompose prior to melting) or have a melting point greater than 300° C.or, alternatively stated, a melting/decomposition temperature of greaterthan 300° C.

The organic nucleating agents are preferably well dispersed in the hdpepolyethylene composition of this invention. The amount of nucleatingagent used is comparatively small—from 100 to 3000 parts by million perweight (based on the weight of the polyethylene) so it will beappreciated by those skilled in the art that some care must be taken toensure that the nucleating agent is well dispersed. It is preferred toadd the nucleating agent in finely divided form (less than 50 microns,especially less than 10 microns) to the polyethylene to facilitatemixing. This type of “physical blend” (i.e. a mixture of the nucleatingagent and the resin in solid form) is generally preferable to the use ofa “masterbatch” of the nucleator (where the term “masterbatch” refers tothe practice of first melt mixing the additive—the nucleator, in thiscase—with a small amount of hdpe resin—then melt mixing the“masterbatch” with the remaining bulk of the hdpe resin).

Examples of high performance organic nucleating agents which may besuitable for use in the present invention include the cyclic organicstructures disclosed in U.S. Pat. No. 5,981,636 (and salts thereof, suchas disodium bicyclo[2.2.1]heptene dicarboxylate); the saturated versionsof the structures disclosed in U.S. Pat. No. 5,981,636 (as disclosed inU.S. Pat. No. 6,465,551; Zhao et al., to Milliken); the salts of certaincyclic dicarboxylic acids having a hexahydrophtalic acid structure (or“HHPA” structure) as disclosed in U.S. Pat. No. 6,559,971 (Dotson etal., to Milliken); and phosphate esters, such as those disclosed in U.S.Pat. No. 5,342,868 and those sold under the trade names NA-11 and NA-21by Asahi Denka Kogyo. Preferred nucleators are cylic dicarboxylates andthe salts thereof, especially the divalent metal or metalloid salts,(particularly, calcium salts) of the HHPA structures disclosed in U.S.Pat. No. 6,559,971. For clarity, the HHPA structure generally comprisesa ring structure with six carbon atoms in the ring and two carboxylicacid groups which are substituents on adjacent atoms of the ringstructure. The other four carbon atoms in the ring may be substituted,as disclosed in U.S. Pat. No. 6,559,971. A preferred example is1,2-cyclohexanedicarboxylic acid, calcium salt (CAS registry number491589-22-1).

Other Additives

The hdpe may also contain other conventional additives, especially (1)primary antioxidants (such as hindered phenols, including vitamin E);(2) secondary antioxidants (especially phosphites and phosphonites); and(3) process aids (especially fluoroelastomer and/or polyethylene glycolbound process aid).

Film Extrusion Process

Blown Film Process

The extrusion-blown film process is a well known process for thepreparation of plastic film. The process employs an extruder whichheats, melts and conveys the molten plastic and forces it through anannular die. Typical extrusion temperatures are from 330 to 500° F.,especially 350 to 460° F.

The polyethylene film is drawn from the die and formed into a tube shapeand eventually passed through a pair of draw or nip rollers. Internalcompressed air is then introduced from the mandrel causing the tube toincrease in diameter forming a “bubble” of the desired size. Thus, theblown film is stretched in two directions, namely in the axial direction(by the use of forced air which “blows out” the diameter of the bubble)and in the lengthwise direction of the bubble (by the action of awinding element which pulls the bubble through the machinery). Externalair is also introduced around the bubble circumference to cool the meltas it exits the die. Film width is varied by introducing more or lessinternal air into the bubble thus increasing or decreasing the bubblesize. Film thickness is controlled primarily by increasing or decreasingthe speed of the draw roll or nip roll to control the draw-down rate.

The bubble is then collapsed into two doubled layers of film immediatelyafter passing through the draw or nip rolls. The cooled film can then beprocessed further by cutting or sealing to produce a variety of consumerproducts. While not wishing to be bound by theory, it is generallybelieved by those skilled in the art of manufacturing blown films thatthe physical properties of the finished films are influenced by both themolecular structure of the polyethylene and by the processingconditions. For example, the processing conditions are thought toinfluence the degree of molecular orientation (in both the machinedirection and the axial or cross direction).

A balance of “machine direction” (“MD”) and “transverse direction”(“TD”—which is perpendicular to MD) molecular orientation is generallyconsidered most desirable for key properties associated with theinvention (for example, Dart Impact strength, Machine Direction andTransverse Direction tear properties).

Thus, it is recognized that these stretching forces on the “bubble” canaffect the physical properties of the finished film. In particular, itis known that the “blow up ratio” (i.e. the ratio of the diameter of theblown bubble to the diameter of the annular die) can have a significanteffect upon the dart impact strength and tear strength of the finishedfilm.

The above description relates to the preparation of monolayer films.Multilayer films may be prepared by 1) a “co-extrusion” process thatallows more than one stream of molten polymer to be introduced to anannular die resulting in a multi-layered film membrane or 2) alamination process in which film layers are laminated together. Thefilms of this invention are preferably prepared using the abovedescribed blown film process.

An alternative process is the so-called cast film process, wherein thepolyethylene is melted in an extruder, then forced through a linear slitdie, thereby “casting” a thin flat film. The extrusion temperature forcast film is typically somewhat hotter than that used in the blown filmprocess (with typically operating temperatures of from 450 to 550° F.).In general, cast film is cooled (quenched) more rapidly than blown film.

Further details are provided in the following examples.

EXAMPLES Example 1

Screening tests for the efficiency of a high efficiency organicnucleating agent in different hdpe barrier film compositions wereconducted on a blown film line manufactured by Battenfeld GloucesterEngineering Company of Gloucester, Mass. This blown film line has astandard output of more than 100 pounds per hour and is equipped with a50 horsepower motor. The extender screw has a 2.5 mil diameter and alength/diameter (L/D) ratio of 24/1.

The blown film bubble is air cooled. Typical blow up ratio (BUR) forbarrier films prepared on this line are from 1.5/1 to 4/1. An annulardie having a gap of 85 mils was used for these experiments.

The films of this example were prepared using a BUR aiming point of 2/1and a film thickness aiming point of 1.5 mils.

The “high efficiency” nucleating agent used in this example was a saltof a cyclic dicarboxylic acid, namely the calcium salt of 1,2cyclohexanedicarboxylic acid (CAS Registry number 491589-22-1, referredto in these examples as “nucleating agent 1”).

Water Vapor Transmission Rate (“WVTR”, expressed as grams of water vaportransmitted per 100 square inches of film per day at a specified filmthickness (mils), or g/100 in²/day) was measured in accordance with ASTMF1249-90 with a MOCON permatron developed by Modern Controls Inc. atconditions of 100° F. (37.8° C.) and 100% relative humidity. A control(comparative) experiment was conducted using a single low melt indexhdpe resin having a melt index of about 1.2 grams/10 minutes, a densityof 0.962 g/cc and a molecular weight distribution, Mw/Mn, of 4.9 (anethylene homopolymer, sold under the trademark SCLAIR™ 19G (“19G resin”)by NOVA Chemicals Inc. (“NCI”) of Pittsburgh, Pa.).

Table 1 illustrates that a film prepared from the 19G resin in theabsence of the nucleator had an MVTR value of 0.2084 g/100 in²/day(film 1) and that the nucleating agent improved the MVTR to 0.1906 g/100in²/day (film 2). This illustrates that nucleating agent 1 is an“organic barrier nucleating agent” that may be used to improve the MVTRperformance of barrier film.

Films 3-6 were prepared by blending 85 weight % of the 19G with 15% ofresins having a high melt index, in the presence and absence of thenucleating agent 1.

Comparative films 3 and 4 were prepared using a hdpe homopolymer resinsold under the trademark SCLAIR™ 2907 as a (comparative) component b).This resin has a melt index of only 4.9 grams/10 minutes (and,accordingly, the melt index ratio of the two hdpe resins is only4.2/1.2, or less than 4/1). The density of 2907 resin is typically 0.960g/cc. As shown in Table 1, a film prepared with this blend in theabsence of a nucleating agent had an MVTR of 0.1851 g/100 in²/day(comparative film 3) and the nucleating agent improved this value to0.1720 g/100 in²/day—an improvement of only 0.0131 g/100 in²/day.

Inventive film 6 and comparative film 5 were prepared using an hdpecomposition prepared by melt blending 85 weight % of the 19G resin with15 weight % of an hdpe homopolymer resin sold under the trademarkSCLAIR™ 79F by NCI as component b). This 79F resin had a melt index of18 grams/10 minutes, a density of 0.963 g/cc and a molecular weightdistribution of 2.7. The overall melt index (I₂) of the blend wasestimated to be 1.8 grams/10 minutes.

As shown in Table 1, comparative film 5 (prepared from the 85/15 blendof the 19G and 79F hdpe resins, in the absence of nucleating agent 1)had an MVTR value of 0.1955 g/100 in²/day.

Inventive film 6, prepared from the hdpe composition of film 5 plus 1000ppm of the nucleating agent, had an MVTR value of 0.1525 g/100 in²/day(which represents an improvement of more than 20% over the MVTR value offilm 5).

Table 1 also illustrates data which describe the properties of barrierfilm prepared from an experimental hdpe homopolymer resin. Thisexperimental resin was prepared in a dual reactor solutionpolymerization process in accordance with the disclosure of publishedU.S. patent application 20060047078 (Swabey et al.). The experimentalresin (EXP in Table 1) had a melt index, I₂, of 1.2 grams/10 minutes, adensity of 0.967 g/cc and a molecular weight distribution, Mw/Mn, of8.9. The EXP resin had two distinct fractions which varied according tomolecular weight. The low molecular weight fraction (or component a))was about 55 weight % of the total composition and had a melt index, I₂,which was estimated to be greater than 5000 grams/10 minutes. The highmolecular weight fraction was about 45 weight % of the total compositionand had a melt index which was estimated to be less than 0.1 grams/10minutes.

As noted above, melt index (I₂) is generally inversely proportional tomolecular weight for polyethylene resins. This was confirmed forhomopolymer hdpe resins having a narrow molecular weight distribution(of less than 3) by preparing a plot of log (I₂) versus log (weightaverage molecular weight, Mw). In order to prepare this plot, the meltindex (I₂) and weight average molecular Mw) of more than 15 differenthomopolymer hdpe resins was measured. These homopolymer hdpe resins hada narrow molecular weight distribution (less than 3) but had differentMw—ranging from about 30,000 to 150,000. (As will be appreciated bythose skilled in the art, it is difficult to obtain reproducible I₂values for polyethylene resins having a molecular weight which isoutside of this range).

A log/log plot of these I₂ and Mw values was used to calculate thefollowing relation between I₂ and Mw for such homopolymer hdpe resins:

I ₂=(1.774×10⁺¹⁹)×(Mw ^(−3.86)).

Extrapolation (based on the above relation) was used to estimate the I₂values of component a) and component b) of the EXP resin. That is, themolecular weight of component a) and component b) was measured and theMw values were used to estimate the I₂ values. It will be appreciated bythose skilled in the art that it can be difficult to physically blendthese hdpe blend components (due to the very different viscosities ofthese hdpe blend components). Accordingly, solution blending or anin-situ blending (i.e. prepared by a polymerization process) arepreferred methods to prepare such hdpe compositions. As shown in Table1, (comparative) film 7, prepared from this EXP resin had an MVTR of0.1594 grams/10 minutes. Inventive film 8 was made with an hdpecomposition prepared by adding 1000 ppm of the nucleating agent to theEXP resin.

Example 2 Comparative

Barrier films were prepared with the inventive hdpe blend compositionsused in experiment 6 of Example 1 (i.e. 85/15 of the aforedescribed 19Gand 79F resins) with other nucleating agents.

The films were prepared on a smaller film line manufactured by MacroEngineering and Technology of Mississauga, Ontario, Canada. The line wasoperated with an annular die having a die gap of 100 mls; a BUR aimingpoint of 2:1 and a film thickness aiming point of 1.5 mils.

The data in Table 2 illustrate that neither talc nor DBS are suitablefor use in this invention.

TABLE 1 HDPE Composition Component Nucleating a) Component b) Agent 1WVTR (g/ Film (wt %) (wt %) (ppm) 100 in²/day) 1-c — “19G (100%)” —0.2084 2-c — “19G (100%)” 1000 0.1906 3-c 2907 (15%) “19G (85%)” —0.1851 4-c 2907 (15%) “19G (85%)” 1000 0.1720 5-c 79F (15%) “19G (85%)”— 0.1955 6 79F (15%) “19G (85%)” 1000 0.1525 7-c “EXP” — 0.1594 8 “EXP”1000 0.0749 Notes: “19G” = SCLAIR ™ 19G (I₂ = 1.2 grams/10 minutes,density = 0.962 g/cc) “2907” = SCLAIR ™ 2907 (I₂ = 4.9 grams/10 minutes,density = 0.960 g/cc) “79F” = SCLAIR ™ 79F (I₂ = 18 grams/10 minutes,density = 0.963 g/cc) EXP = experimental resin (described above) (I₂ =1.2 grams/10 minutes, density = 0.967 g/cc)

TABLE 2 Nucleating Agent WVTR Film (ppm) (g/100 in²/day) 10 None 0.244511 Talc (2500 (ppm) 0.2503 12 DBS (ppm) 0.3836 13 Organic Nucleating0.1574 Agent (1000 ppm) Notes: Organic nucleating agent 1 was the sameas used in inventive films 2, 4, 6 and 8 of example 1. The hdpecomposition used in all experiments was that of experiment 6 of example1 (i.e. 85 weight % SCLAIR ™ 19G resin and 15 weight % SCLAIR ™ 79Fresin). The “DBS” nucleating agent is a dibenzylidene sorbital estersold under the trademark Irgaclear ™ by Ciba. The talc was sold underthe trademark Cimpact ™ 699 and was reported to have an average particlesize of 1.5 microns and an aspect ratio of 5:1.

What is claimed is:
 1. A barrier film comprising at least one extrudedpolyethylene layer, wherein said at least one extruded polyethylenelayer comprises: I) an organic barrier nucleating agent and II) a highdensity polyethylene blend composition comprising: II-i) from 5 to 60weight % of at least one high density polyethylene blend component a)having a high melt index, I₂; and II-ii) from 95 to 40 weight % of atleast one high density polyethylene blend component b) having a low meltindex, I₂′, wherein: w) said organic barrier nucleating agent is addedin an amount of from 100 to 3000 parts per million based on the weightof said high density polyethylene blend composition; x) each of saidblend component a) and blend component b) has a density of from 0.950 to0.975 g/cc; y) the melt index, I₂, of said high density polyethyleneblend composition is from 0.5 to 10 grams/10 minutes; and z) the I₂ratio, obtained by dividing the I₂ value of said blend component a) bythe I₂′ value of said blend component b) is greater than 10/1.
 2. Thebarrier film of claim 1 wherein said high density polyethylene blendcomposition comprises from 10 to 40 weight % of said component a) andfrom 90 to 60 weight % of said component b).
 3. The barrier film ofclaim 1 wherein said high density polyethylene blend compositioncomprises from 20 to 40 weight % of said component a) and from 80 to 60weight % of said component b).
 4. The barrier film of claim 1 whereinsaid blend component a) is further characterized by having a molecularweight distribution, Mw/Mn, of from 2 to
 4. 5. The barrier film of claim1 wherein said high density polyethylene blend composition has a densityfrom 0.955 to 0.965 g/cc.
 6. The barrier film of claim 1 wherein saidhigh density polyethylene blend composition has a melt index, I₂, offrom 0.8 to 8 grams/10 minutes.
 7. The barrier film of claim 1 whereinsaid nucleating agent has a melting/decomposition temperature of greaterthan 300° C.
 8. The barrier film of claim 6 wherein said nucleatingagent is a salt of a dicarboxylic acid.
 9. The barrier film of claim 8wherein said dicarboxylic acid is a cyclic dicarboxylic acid having ahexahydrophtalic structure.
 10. The barrier film of claim 9 wherein saidsalt is selected from the group consisting of calcium and zinc salts.11. A process to prepare a barrier film for food packaging, said processcomprising the film extrusion of a composition comprising: I) an organicbarrier nucleating agent; and II) a high density polyethylene blendcomposition comprising: II-i) from 5 to 60 weight % of at least one highdensity polyethylene blend component a) having a high melt index, I₂;and II-ii) from 95 to 40 weight % of at least one high densitypolyethylene blend component b) having a low melt index, I₂′, wherein:w) said organic barrier nucleating agent is added in an amount of from100 to 3000 parts per million based on the weight of said high densitypolyethylene blend composition; x) each of said blend component a) andblend component b) has a density of from 0.950 to 0.975 g/cc; y) themelt index, I₂, of said high density polyethylene blend composition isfrom 0.5 to 10 grams/10 minutes; and z) the I₂ ratio, obtained bydividing the I₂ value of said blend component a) by the I₂′ value ofsaid blend component b) is greater than 10/1.
 12. The process of claim11 wherein said high density polyethylene blend composition comprisesfrom 10 to 40 weight % of said component a) and from 90 to 60 weight %of said component b).
 13. The process of claim 11 wherein said blendcomponent a) is further characterized by having a molecular weightdistribution, Mw/Mn, of from 2 to
 4. 14. The process of claim 13 whereinsaid nucleating agent is a salt of a dicarboxylic acid.
 15. The processof claim 11 wherein conducted at a blow up ratio of from 1.5/1 to 4/1.